Mechanisms of Shoulder Injury



Mechanisms of Injury

Physical Therapy

Joint Replacement

Cartilage Repair

Contact Info

| Home | Rotator Cuff | Shoulder Separation and Dislocation | Cartilage | References |

Rotator Cuff

The most common shoulder injury in sports occurs in the rotator cuff, the group of muscles, tendons, and ligaments that control the shoulder's mobility. There are multiple injuries associated with the rotator cuff, which are all grouped into the common term “rotator cuff tear.”

Sports-Related Rotator Cuff Tear

Sport related injury to the rotator cuff is can be broken down in to two categories, chronic and acute. Chronic tear is far more common, and is usually a result of a tear that occurs in sports that entail a follow-through action such as repetitive throwing, overhead strokes, and swimming. The muscle or tendon tear is usually a result of joint imbalance that may be associated with looseness in the front of the shoulder caused by aweakness in supporting ligaments. Acute tears occur in instances of intense stress or pressure, such as lifting something extremely heavy. Here is an overview of sport specific injuries to the shoulder.


The shoulder injury most common in swimmers is dubbed “swimmer's shoulder.” Swimmer's shoulder is caused by impingement of the soft tissue in the shoulder against the coracoacromial arch, or the acromion. This inflammatory condition is caused by the overhead free-style stroke. There are two phases of the stroke that overextend the shoulder muscles, and result in joint imbalance:

•  The Pull-Through Phase: This phase of the freestyle stroke begins when the hand enters the water and ends when the arm has completed pulling through the water and is about to exit the water. When the hand enters the water, the shoulder is in a position of horizontal adduction which pinches the head of the bicep against the anterior part of the acromion. The difference between the swimming stroke and simply raising one's arm into an outstretched position is that the swimmer must fight the resistance provided by the water in order to release the bicep muscle from the joint.

•  The Recovery Phase: This stage of the stroke begins when the arm exits the water and lasts until the hand enters the water again. When a swimmer fatigues, tired muscles make this phase more difficult. The muscles in the rotator cuff which work to rotate and depress the head of the humerus against the cartilage in the joint, or glenoid, become less efficient. When these muscles do not work properly, or become imbalanced, the supraspinatus tendon becomes pinched between the humerus and the acromion.

These injuries can be incurred separately or in tandem. They both range in severity. Swimmers shoulder is caused by multiple factors:

•  Poor Stroke Technique : Improper stroke technique can result in muscle imbalance, and consequently, joint imbalance.

•  Unilateral Breathing : Although most swimmers are most comfortable breathing in just one direction, this is the biggest cause of muscle imbalance in swimmers.

•  Overuse : Ultimately, these are chronic injuries that occur because of repeated stress to the muscles, tendons and joints.

Baseball, Overhead Throwing Sports

Baseball injuries illustrate another form of chronic rotator cuff injury. Pitchers are most susceptible, due to the force with which they throw, and the constant repetition of motion. However, all players have the same throwing motion that results in the injury, and are thus susceptible.

The baseball throwing motion has been studied in detail and can be divided into 5 stages.

•  Wind-up : rotator cuff muscles are inactive during this initial stage.

•  Early cocking stage : involves external shoulder rotation.

•  Late cocking stage : the rotator cuff muscles are very active during this stage, especially the subscapularis, which contracts and acts as a dynamic stabilizer

•  Acceleration stage : begins with internal rotation of the humerus and ends with release of the baseball. The muscles of the rotator cuff are basically inactive.

•  Follow-through : during this phase, the rotator cuff muscles are most active. The supraspinatus contracts to decelerate internal rotation of the limb.

Based on this study of the throwing motion, it is clear that the two stages that affect the rotator cuff are the late cocking stage and the follow through. As with swimmer's shoulder, injury may occur due to improper motion, muscle imbalance, or simple overuse. It is important to note that two different tendons in the rotator cuff are impinged by the throwing motion, and a tear in either tendon (the subscapularis or the supraspinatus) constitutes a rotator cuff tear.

The baseball throwing motion can be applied to any other sport that involves throwing, such as football, and the javelin. Furthermore, the stages of the throwing motion parallel the motion in overhead racquet sports (tennis, squash, racquetball) and the spiking motion in volleyball. The sport that has the highest occurrence of rotator cuff injuries is water polo, as involves the risk associated with swimming as well as the risks involved with repeated throwing.

Non-Sport Related Rotator Cuff Injury

Not all injuries are due to repeated motion or chronic stress. Impingement syndrome is the most common non-sport related rotator cuff injury. If the supraspinatus tendon passes beneath the acromion, the bone on top of the shoulder, the tendon and the lubricating tissue, or the bursa, is then pinched when the arm is raised into a forward or upward position. Repetitive impingement can make the tendons and the bursa inflamed, resulting in the disease referred to as impingement syndrome.

Factors that put people at a high risk for impingement syndrome:

•  Shape/Thickness of the acromion : as discussed in the biology section, the shoulder is a ball and socket type joint, the acromion being the socket, and the humeral head being the ball. In some people the space between the undersurface of the acromion and the humeral head is very narrow, and thus is more likely to pinch the supraspinatus tendon.

•  Bone Spur : Some people have bone spurs on the front of the acromion, which increases the risk of impingement syndrome.

•  Muscle Imbalance : Either by injury or atrophy, the muscles in the shoulder may become imbalanced, which affects the shoulder motion, especially in the forward direction. Any change in motion increases the risk of impingement.

The symptoms of impingement syndrome are as follows:

•  Pain : Impingement syndrome begets an aching pain in both the front of the shoulder and also the outer side of the upper arm. The pain will increase on contact as it is tender to the touch, and also certain movements will result in sharp searing pain.

•  Weakness : Due to imbalance, and improper shoulder movement, the shoulder will become weak.

•  Lack of Mobility : Some movements may pinch the tendon in such a way that the shoulder will not be able to rotate sufficiently to allow the arm to complete the action. Other motions may simply be too painful.

Impingement syndrome breaks down the supraspinatus tendon near the attachment to the humerus bone. In most severe cases, the tendon may pull away from the bone completely.


Rotator cuff injuries are most commonly treated with physical therapy and strengthening exercises, but severe cases require surgical intervention. Indications for operative treatment of rotator cuff disease include partial-thickness or full-thickness tears in an active individual who does not improve pain and/or function within 3-6 months with a supervised rehabilitation program.

Back to top...

Created by: Blair Smith, Connie Lee, Daniel Solomon, Matthew Whitson and Stephanie Chang
Date last modified: 5/2/2004